Stainless steel forgings are widely used in industry because of their excellent corrosion resistance. However, under certain conditions,
forgings may still suffer from corrosion. The corrosion mechanism mainly includes chemical corrosion and electrochemical corrosion. These two corrosion mechanisms of
stainless steel forgings and their influencing factors will be discussed in detail below.
Chemical corrosion mainly refers to the corrosion phenomenon caused by the reaction of stainless steel with chemical substances, mainly including the following aspects.
The oil, dust and acid, alkali, salt and other chemicals attached to the surface of stainless steel will be converted into corrosive media under certain conditions. These pollutants will chemically react with certain components in stainless steel, resulting in the destruction of the passivation film, which will lead to chemical corrosion. Especially in a humid or chemical environment, this corrosion phenomenon is more obvious.
There may be various scratches on the surface of stainless steel during processing or use, which will destroy the passivation film of stainless steel and reduce its protective ability. When the passivation film is destroyed, the corrosion resistance of stainless steel is reduced, and it is easy to react with the surrounding chemical medium, resulting in chemical corrosion. Corrosion on scratches is usually more severe than in other areas because these areas lack protective passivation film.
In the pickling passivation process, if the cleaning is not thorough, it will cause the residual liquid to form a corrosive medium on the stainless steel surface. The residual liquid after pickling is in direct contact with the stainless steel parts, which will cause corrosion on the surface, resulting in corrosion. The chemical composition in the residue reacts with the stainless steel, further accelerating the corrosion process.
Electrochemical corrosion is the corrosion caused by the electrochemical reaction between metals or metal parts with different potentials.
When stainless steel is in contact with carbon steel parts, it may cause scratches and form galvanic cells. The scratch area of carbon steel forms an electrochemical corrosion battery with the surface of stainless steel, so that the corrosive medium accelerates the corrosion process in these areas. This electrochemical corrosion is caused by the difference in potential between two different metals, carbon steel as the anode, and stainless steel as the cathode, resulting in corrosion of stainless steel.
The slag and splash produced during the cutting process, if attached to the stainless steel surface, will form a galvanic cell with the corrosive medium. These adhesion substances may contain rust or other corrosive substances, forming an electrochemical corrosion environment with stainless steel, resulting in corrosion phenomena. Especially in the high temperature environment, this corrosion phenomenon will be more obvious.
In the flame heated area, the metallographic structure of the stainless steel will change, making the chemical composition of these areas uneven. This change in composition will cause the stainless steel to form a galvanic cell in the corrosive medium, causing electrochemical corrosion. Changes in the composition of the heating area will affect the overall corrosion resistance of the stainless steel, thus accelerating the corrosion process.
Physical defects that may occur in the welding process (such as edge biting, pores, cracks, etc.) and chemical defects (such as coarse grains, chromium-poor grain boundaries, segregation, etc.) will lead to electrochemical corrosion in the welding area. These defects cause the welding area to form a galvanic cell with the surrounding corrosive medium, which leads to electrochemical corrosion and accelerates the occurrence of corrosion.
The chemical defects of stainless steel itself (such as uneven composition, sulfur, phosphorus impurities, etc.) and surface physical defects (such as loose, sand holes, cracks, etc.) will lead to the formation of galvanic cells with corrosive media. These defects cause electrochemical corrosion of stainless steel in corrosive media, thereby reducing its corrosion resistance.
Poor pickling passivation results in uneven or thin passivation film on stainless steel surface. This uneven passivation film can not effectively protect the surface of stainless steel, and is easy to form electrochemical corrosion, resulting in corrosion.
If the pickling passivation residue is not thoroughly cleaned, it will chemically react with the stainless steel surface to produce corrosive substances. These corrosive substances will form an electrochemical corrosion environment, resulting in accelerated corrosion of the stainless steel surface.
Stress concentration area is prone to stress corrosion, this corrosion phenomenon occurs at the stress concentration, usually due to local stress caused by excessive corrosion. Stress corrosion will cause the mechanical properties of stainless steel material to decline, thus affecting its corrosion resistance.
Even though stainless steel is known for its strong resistance to corrosion, it's still important to watch out for conditions that can lead to corrosion during its use and processing. Proper care and attention are needed to avoid these factors and maintain the material’s durability and effectiveness over time.
